1. Technical Field
The present invention relates to a method and circuit for illuminating an incandescent lamp which suppresses flow of an inrush current, which would otherwise arise at the time of illumination of the incandescent lamp.
2. Related Art
In relation to a related-art circuit for illuminating an incandescent lamp, such as a vehicle-installed lamp, a switch such as a mechanical switch or a semiconductor switch is provided in the illumination circuit. Illumination and extinguishing of the incandescent lamp are effected, by means of activating or deactivating the switch. As described in the Unexamined Japanese Patent Application Publication No. Hei8-171813, there has been proposed that a thermistor possessing a negative characteristic be inserted into the illumination circuit along with the switch, thereby suppressing flow of an inrush current into the incandescent lamp at the beginning of energization of the incandescent lamp.
[Problems to be Solved]
However, in a situation in which only the switch is inserted into the illumination circuit, when the incandescent lamp is not illuminated, a filament of the incandescent lamp is cold and has low resistance. Hence, a large inrush current flows into the incandescent lamp at the beginning of energization resulting from activation of a switch, thereby shortening the life of the filament. If a large inrush current flows into the incandescent lamp, fluctuations will arise in a supply voltage, which may adversely affect another electronic device.
When a thermistor having a negative characteristic is inserted into the illumination circuit, flow of a large inrush current into the incandescent lamp is suppressed, because the thermistor has large resistance at the beginning of energization. After lapse of a predetermined period of time, the resistance of the thermistor decreases. Hence, a rated current flows into the incandescent lamp and enters a normal illuminated state. However, even in a normal illuminated state, the thermistor has predetermined resistance, and hence an electric current is wastefully dissipated.
The present invention is conceived in view of the foregoing circumstances and aims at providing a method and circuit for illuminating an incandescent lamp which enable effective suppression of flow of an inrush current into the incandescent lamp without involvement of wasteful dissipation of power.
[Means for Solving the Problems]
To achieve the object, the present invention provides An incandescent lamp illumination method, wherein an incandescent lamp and a semiconductor switch element having a control terminal are connected in series with a power supply, and the incandescent lamp is illuminating by means of supplying a control signal to the control terminal to thereby bring the semiconductor switch element into conduction, the method comprising the step of: suppressing conduction of the semiconductor switching element, by means of reducing a voltage level of the control signal to be supplied to the control terminal so as to become lower than a voltage level obtained at the time of normal illumination of the incandescent lamp, at the beginning of energization of the incandescent lamp.
According to the method, at the beginning of conduction, filament resistance of the incandescent lamp is low. At this time, a voltage level of a control signal supplied to the control terminal of the semiconductor switch element is lower than that obtained at the time of normal illumination of the incandescent lamp, so that conduction of the semiconductor switch element is suppressed, to thereby hinder flow of a large current. Consequently, flow of an inrush current into the incandescent lamp is suppressed. When a predetermined period of time has elapsed, the control signal to be supplied to the control terminal is set to the voltage level obtained at the time of normal illumination of the incandescent lamp, thus bringing the incandescent lamp into a normal illuminated state.
Preferably, the voltage level of the control signal is gradually increased, thereby gradually bringing the semiconductor switch into conduction.
According to the method, the voltage level of the control signal is gradually increased, thereby gradually bringing the semiconductor switch element into conduction. At the beginning of energization, the conduction of the semiconductor switch is suppressed, thereby inhibiting flow of a great electric current. In contrast, when a predetermined period of time has elapsed, the voltage level of the control signal supplied to the control terminal becomes large, thereby bringing the semiconductor switch element into normal continuity. Thus, the incandescent lamp is brought into a normal illuminated state.
The present invention provides an incandescent lamp illumination circuit, wherein an incandescent lamp and a semiconductor switch element having a control terminal are connected in series with a power supply, and the incandescent lamp is illuminating by means of supplying a control signal to the control terminal to there by bring the semiconductor switch element into conduction, the circuit comprising: control signal supply means for supplying a control signal to the control terminal; and signal level control means for suppressing energization of the semiconductor switch element, by means of reducing a voltage level of the control signal to be supplied to the control terminal so as to become lower than a voltage level obtained at the time of normal illumination of the incandescent lamp, at the beginning of energization of the incandescent lamp.
By means of this configuration, at the beginning of energization, the incandescent lamp has low filament resistance. At this time, a voltage level of the control signal supplied to the control terminal of the semiconductor switch element is lower than that obtained at the time of normal illumination of the incandescent lamp. Hence, energization of the semiconductor switch is suppressed, thereby inhibiting flow of a large current. Consequently, flow of an inrush current into the incandescent lamp is suppressed. When a predetermined period of time has elapsed, a control signal to be supplied to the control terminal is brought to a voltage level obtained at the time of normal illumination of the incandescent lamp, thereby bringing the incandescent lamp into a normal illuminated state.
Preferably, the signal level control means gradually brings the semiconductor switch element into conduction, by means of gradually increasing a voltage level of the control signal.
By means of the configuration, the voltage level of the control signal is gradually increased, thereby gradually bringing the semiconductor switch element into conduction. At the beginning of energization, the conduction of the semiconductor switch is suppressed, thereby inhibiting flow of a great electric current. In contrast, when a predetermined period of time has elapsed, the voltage level of the control signal supplied to the control terminal becomes large, thereby bringing the semiconductor switch element into normal continuity. Thus, the incandescent lamp is brought into a normal illuminated state.
Preferably, the signal level control means is constituted of a time-constant circuit comprising a resistance element and a capacitor; the resistance element is interposed between the control signal supply means and the control terminal; the capacitor is between the control terminal and a reference potential.
By means of the configuration, electric charge is stored in the capacitor in accordance with a time constant-which is defined by a product of the resistance of the resistance element and the electrostatic capacitance value of the capacitor-and hence the voltage level of the control signal increases gradually. Consequently, the semiconductor switch element is gradually brought into the state of conduction. At the beginning of energization, the conduction of the semiconductor switch element is suppressed, thereby inhibiting inflow of a large inrush current into the incandescent lamp.
Preferably, the control signal supply means outputs a pulse signal, and a current direction regulation element is interposed between the control terminal and the capacitor for enabling passage of an electric current during only an ON period of the pulse signal.
By means of the configuration, electric charge is stored in the capacitor via the current direction regulation element at the time of an ON period of the pulse signal; that is, when the pulse signal is at a high level. At the time of an OFF period of the pulse signal; that is, when the pulse signal is at a low level, presence of the current direction regulation element prevents discharge of the electric charge stored in the capacitor. Hence, the potential of the control terminal reaches a predetermined level within a predetermined period of time without fail.
Preferably, the current direction regulation element is a diode.
Preferably, a discharge resistance element is connected in parallel with the capacitor.
By means of the configuration, supply of the control signal to the control terminal is stopped, thereby bringing the incandescent lamp into an extinguished state from an illuminated state. At this time, the electric charge stored in the capacitor is discharged via the discharge resistance element. Consequently, even when the control signal is again supplied to the control terminal, flow of an inrush current into the incandescent lamp is suppressed without fail. So long as discharge of electric charge is set so as to finish before the filament of the incandescent lamp becomes cold, flow of a large inrush current can be prevented by the electrical resistance of the filament still having a high value even when the control signal is supplied to the control terminal before completion of discharge.